1. Field of the Invention
Example embodiments of the present invention relate to an organic thin film transistor and to a method for fabricating the same. More particularly, example embodiments of the present invention relate to a method for fabricating an organic thin film transistor with improved (e.g., higher) charge carrier mobility. According to example embodiments of the present invention, the organic thin film transistor may include a substrate, a gate electrode, a gate insulating film, metal oxide source/drain electrodes and an organic semiconductor layer. In example embodiments of the present invention, the metal oxide source/drain electrodes may be surface-treated with a self-assembled monolayer (SAM) forming compound containing a sulfonic acid group so that the hydrophobic quality, charge carrier mobility and/or work function of a metal oxide constituting the source/drain electrodes may be increased.
2. Description of the Related Art
With the advent of polyacetylenes as conjugated organic polymers exhibiting semiconductor characteristics, organic semiconductors have been investigated as electrical and electronic materials in a wide variety of applications, e.g., functional electronic and optical devices. Because organic semiconductors may lend themselves to various synthetic processes, may be easier to mold into fibers and films, and may exhibit superior flexibility, higher conductivity organic semiconductors may provide a way to lower manufacturing costs.
Organic thin film transistors may have advantages in that semiconductor layers may be formed by printing processes at ambient pressure instead of being formed by conventional silicon processes such as plasma-enhanced chemical vapor deposition (CVD). Also, if needed, the overall fabrication procedure may be achieved by roll-to-roll processes using plastic substrates, which may be economically advantageous over silicon thin film transistors. Among other possible uses, thin film transistors may be integrated into active displays and plastic chips (or other suitable chip material) for use in smart cards and inventory tags.
Nevertheless, organic thin film transistors may suffer from having a low charge carrier mobility, high driving voltage and/or high threshold voltage in comparison with silicon thin film transistors. But organic thin film transistors having a charge carrier mobility of about 0.6 cm2·V−1·sec−1 using, for example, pentacene in an organic thin film transistor may still be put to practical use. The charge carrier mobility of the organic thin film transistor, however, may still be unsatisfactory. Further, there may be some disadvantages of organic thin film transistors because they may require a driving voltage of about 100 V or more and a threshold voltage of about 50 times higher than that required of silicon thin film transistors.
Sometimes when pentacene is used as the organic semiconductor material in a bottom-contact or top-gate organic thin film transistor, the pentacene is prone to deposit on a gate insulating layer rather than on source/drain electrodes and may have a relatively high work function compared to metal source/drain electrodes. Consequently, a Schottky barrier may be formed between the source/drain electrodes and the organic semiconductor layer, which may lower the charge carrier mobility of the organic thin film transistor.
According to a conventional process, organic thin film transistors may be produced by first treating the exposed surface of source/drain electrodes with a self-assembled monolayer compound containing a thiol functional group before deposition of an organic semiconductor layer. However, the monolayer compound may become bound to metal surfaces, e.g., gold (Au), while remaining unbound to the surface of metal oxides, e.g., indium-tin oxide. Thus, formation of improved thin film transistors comprising metal oxide source/drain electrodes and an organic semiconductor layer by such monolayer process has proved to be somewhat difficult.